The antimicrobial effect of pregnenolone on clinical and reference strains of Escherichia coli and Staphylococcus aureus was tested. Pregnenolone was found to have inhibitory activity against all tested bacterial strains. Slightly higher sensitivity is shown by the strains of S. aureus. When applied directly, pregnenolone has a weak antimicrobial effect due to its very low water solubility, as it is in oleose state because the formulation studied in this experiment contains tocopherol (a fat-soluble vitamin) as a co-solvent. When emulsified with lecithin, as well as with methanol, its solubility in water increases and penetrates over a longer distance in the agar around the points of its application. Applied as an emulsion, it shows significantly higher inhibitory activity against E. coli and S. aureus. Even the non-emulsified version should still be useful in vivo due to the fact that intracellular environments are much more lipophilic than serum, the target of most antimicrobial substances is the intracellular space, and non-emulsified pregnenolone has been shown to have very high intracellular uptake.

The antimicrobial effect of a cream containing extracts of African geranium (Pelargonium sidoides DC.), black elderberry (Sambucus nigra L.), and St. John’s wort (Hypericum perforatum L.) in colloidal nanosilver (AgNPs) at a concentration of 30 ppm, denoted as SILVER STOP® cream (SS® cream), was examined in vitro. The research was performed with Escherichia coli (ATCC and two clinical isolates), Staphylococcus aureus (ATCC and two clinical strains), and Candida albicans (ATCC and two clinical isolates). The agar-gel diffusion method and suspension tests for determination of the time of antimicrobial action of SS® cream were used. SS® cream showed significant antimicrobial activity. The Gram-negative microorganisms tested died in a much shorter time than the Gram-positive ones. In suspension with a density of 104 cells·mL−1, E. coli died for 1 min, the oval fungus C. albicans—after 10 min and S. aureus—after 60 min of exposure to SS® cream. The highest sensitivity was found in E. coli. The curative effect of SILVER STOP® cream was also examined in vivo in dogs with different skin diseases. The results showed successful healing of the diseases and a very good curative effect of the cream.

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